Uber tested autonomous cars in California, Arizona, Pennsylvania, and Ontario, Canada, in all sorts of conditions before operations were suspended in the wake of the fatal crash. Within the next few weeks, it will resume testing only on a one-mile loop between two of its offices in Pittsburgh. Cars won't exceed 25 miles per hour, and won't operate at night or in rain.

But even this lower bar may be difficult for Uber to reach. Over the past few months, cars have reacted slower than human drivers in closed-course tests, and have failed to pass so-called validation tests, the last step before returning to public roads, according to The New York Times. The newspaper cites internal Uber documents and emails, as well as interviews with seven current and former employees, who spoke on the condition of anonymity.

According to the emails, cars were failing to meet performance benchmarks as recently as a few weeks ago. To match the reaction time of a human at 25 mph, cars needed to drive "20 percent slower than a human," Brandon Basso, a director at Uber's Advanced Technologies Group (ATG) autonomous-driving unit, said in a Nov. 1 email.

But a week later, ATG boss Eric Meyhofer declared in an email that testing would resume at 25 mph. He said this would prove that cars were "unequivocally worthy of being back on the road." However, employees interviewed by The New York Times said Meyhofer's decision may have also been an attempt to demonstrate progress to Uber CEO Dara Khosrowshahi.

The employees reportedly worried that Uber was taking shortcuts to meet internal benchmarks, but Uber spokeswoman Sarah Abboud told The New York Times that the company is making safety its top priority.

"As we have said many times before, our return is predicated on successfully passing our rigorous track tests and having our letter of authorization from the Pennsylvania Department of Transportation in hand," Abboud said.

Uber suspended all self-driving car tests on public roads after a vehicle struck and killed a pedestrian in Tempe, Arizona, in March. In addition to the car failing to detect the person, police subsequently reported that the human safety driver was streaming television on her phone at the time.

In July, Uber redeployed test vehicles in Pittsburgh, but not in autonomous mode. The company also scored a $500 million investment from Toyota in August. As part of the deal, Uber will install its autonomous-driving tech in a fleet of Toyota Sienna minivans. But Uber has definitely fallen behind in the autonomous-car race.

Waymo just launched a commercial ride-hailing service using self-driving cars in Arizona, while Ford is undertaking large-scale test programs in Florida, and Washington, D.C. General Motors' Cruise division has netted investments from Honda and Japan's SoftBank (which also invests in Uber). Both GM and Ford have promised to launch production self-driving cars with no manual controls over the next few years.